Rough cut silicon wafers normally require polishing before they are acceptable for use in electronic devices. Silica polishing is a conventional method for the fine surface polishing of silicon wafers. This method involves positioning the wafer on a polishing head which is then positioned above a rotating polishing plate. The polishing plate is covered with a polishing pad (i.e., a composite pad with a polymeric material) and is held there with an adhesive. During polishing, dilute colloidal silica is continuously injected between the polishing plate and the silicon wafer for fine polishing of the wafer.
The electrical performance of finished semiconductor chips can easily be affected by contaminants acquired by the wafers during processing. Such contamination can be in the form of discrete particles and water soluble or dispersed organic and inorganic impurities. That is, the use of silica sols that are contaminated with trace transition metals, alkali and alkaline earth metals, aluminum, and the like have caused difficulties especially when used in wafer polishing.
These contaminating metals are particularly a nuisance when they include Na, K, alkali and alkaline earth metals such as Ca, Mg, and transition metals such as Fe, Cu, Mn, Ni, Zn, and the like. In general, any transition metal from groups IB, IIB, IIIB, IVB, VB, VIB, and group VIII of the Periodic Table of Elements, if present in high enough concentrations, can cause difficulties in the final products manufactured with silica sols containing these contaminants.
Another metal which can cause difficulties, particularly in the manufacture of certain catalysts, is aluminum. If aluminum is present, and particularly if Fe, Ni, Cu, Mn, or Cr are also present, the silica sols often cannot meet the requirements for the final product, whether those requirements include catalysts, in refractories, in investment casts, or in electronic polishing agents used for electronic wafers.
Alkali metals like lithium, sodium, and potassium demonstrate shifts in electrical properties (threshold and flat-band voltages) when incorporated into semiconductor devices. Heavy metals, such as copper, gold and iron, tend to lower minority carrier lifetime, while increasing dark and leakage currents. Tin, nickel and chromium behave similarly except that they also exhibit a lower oxide breakdown voltage.
Even anions like chloride have a detrimental effect on electrical properties through the process of gettering (concentrating) of the heavy metals and the complexing of the alkali metals.
An additional problem of metal contaminants is that many of these substances have much higher diffusivities in both silicon and silicon dioxide than do the more conventional dopants, such as phosphorus and boron. As a result unpredictable electrical properties can exist.
The effect of metal contaminants on silicon wafers has been widely reported in the following articles: Hiramoto et al., "Degradation of Gate Oxide Integrity by Metal Impurities," Japanese Journal of Applied Physics, Part 2 (Letters), Vol. 28, No. 12, pp. 2109-11 (December 1989); Seibt et al., "TEM Study of Metal Impurity Precipitates in the Surface Regions of Silicon Wafers," Defects in Electronic Materials Symposium, Mater. Res. Soc., Pittsburgh, Pa., pp. 215-18 (Nov. 30-Dec. 3, 1987); Hourai, et al., "A Method of Quantitative Contamination with Metallic Impurities of the Surface of a Silicon Wafer," Japanese Journal of Applied Physics, Part 2 (Letters), Vol. 27, No. 12, pp. 2361-3 (December 1988); Corradi et al., "Surface Contamination Detection Below the ppb Range on Silicon Wafers," Journal of Crystal Growth, Vol 89, No. 1, pp. 39-42 (June 1988); Takizawa et al., "Degradation of Metal-Oxide-Semiconductor Devices Caused by Iron Impurities on the Silicon Wafer Surface", Journal of Applied Physics, Vol. 62, No. 12, pp. 4933-5 (Dec. 15, 1987): Honda et al., "Catastrophic Breakdown in Silicon Oxides: the Effect of Fe Impurities a the SiO.sub.2 -Si Interface," Journal of Applied Physics, Vol. 62, No. 5, pp. 1960-3 (Sep. 1, 1987); K. Graff, "Transition Metal Impurities in Silicon and Their Impact on Device Performance," SEMICON/EUROPA 1983, Semiconductor Equipment & Material Institute, Mountain View, Calif., pp. 9-19, (Mar. 8-10, 1983); P.J. Ward, "A Survey of Iron Contamination in Silicon Substrates and its Impact on Circuit Yield," Journal of the Electrochemical Society, Vol. 129, No. 11, pp. 2573-6 (November 1982); and Pearce et al., "Role of Metallic Contamination in the Formation of `Saucer` Pit Defects in Epitaxial Silicon," Journal of Vacuum Science and Technology, Vol. 14, No. 1, pp. 40-3 (January-February 1977).
It is therefore critical to minimize the possibility of metal contamination in or on the silicon wafer prior to device manufacturing. One concern of semiconductor manufacturers is that colloidal silica containing metals will contaminate the wafer surface. Therefore, it is extremely desirable that colloidal silica products be formed with low sodium and metals content.
The preparation of low sodium, low metals silica is well-known. Various attempts have been made to reduce or eliminate sodium and/or metals from the silica source. A few examples are given in U.S. Pat. No. 4,624,800 (Sasaki et al.), issued Nov. 25, 1986; U.S. Pat. No. 3,024,089 (Spencer et al.), issued Mar. 6, 1962; Japanese Patent Application No. 88/285112 (Watanabe et al.), filed Nov. 22, 1988; Stober and Fink, "Controlled Growth of Monodisperse Silica Spheres in the Micron Size Range", Journal of Colloid and Interface Science, Vol. 26, 1968, pp. 62-69; Wagner and Brunner, "Aerosil, Herstellung, Eigenschaften und Verhalten in Organischen Flussigkeiten", Angew. Chem., Vol. 72, No. 19/20, 1960, pp. 744-750; and Iler, "Chemistry of Silica", Wiley Interscience, 1979, p. 359.
The Sasaki et al. patent discloses a method for producing an aqueous low alkali-metal, low alumina silica sol by treatment of a powder silica with acid to remove the metals while applying ultrasonic vibrations. Spencer discloses a process for preparing finely divided metallic oxides by hydrolyzing a compound containing the corresponding metal while in contact with a finely divided carbonaceous carrier on which the oxide is deposited and then separating the oxide from the carbon.
The Stober article discloses a system of chemical reactions which permit the controlled growth of spherical silica particles of uniform size by means of hydrolysis of alkyl silicates and subsequent condensation of silicic acid in alcoholic solutions. Ammonia is used as a morphological catalyst.
Low sodium silica sol products, with or without low metal content, can also be prepared by removal of the counterions using ion exchange and then backadding ammonium hydroxide and ammonium carbonate to form stable products according to the Iler article.
Although colloidal silica is normally used in a once through polishing system, the cost of the silica and chemicals admixed therewith have caused an increased interest in the development of commercially acceptable recirculation systems. Recirculation systems provide fast polishing rates without high temperatures, avoid wafer warping, and substantially reduce the chemical cost of the polishing step. Unfortunately, when colloidal silicas, with or without organic accelerators, are placed in service for prolonged periods of time they exhibit increased microorganism and fungi growth. Bacterial contamination causes discoloration, odors, and makes the colloidal silica unacceptable as a polishing aid in wafer production.
Microorganism and fungi growth in colloidal silica are well known. Various attempts have been made to reduce or eliminate bacterial growth in colloidal silica. A few examples are shown in: U.S. Pat. Nos. 3,336,236 (Michalski), issued Aug. 15, 1967; 3,816,330 (Havens), issued Jun. 11, 1974; 3,860,431 (Payne), issued Jan. 14, 1975; 2,823,186 (Nickerson), issued Feb. 11, 1958; 2,801,216 (Yoder et al.), issued Jul. 30, 1957; 3,046,234 (Roman et al.), issued Jul. 24, 1962; 3,377,275 (Michalski et al.), issued Apr. 9, 1968; 3,148,110 (McGahen), issued Sep. 8, 1964; 4,169,337 (Payne), issued Oct. 2, 1979; 4,462,188 (Payne), issued Jul. 31, 1984; 4,588,421 (Payne), issued May 13, 1986; 4,892,612 (Huff), issued Jan. 9, 1990; and 4,664,679 (Kohyama et al.), issued May 12, 1987.
The Michalski '236 patent discloses a method for protecting aqueous colloidal silica sols from bacterial contamination. This patent suggests that colloidal aqueous silica sols can be protected from bacterial contamination by simply adding sodium chlorite in an amount sufficient to inhibit growth and reproduction of the bacteria. Generally from about 10 parts of sodium chlorite per million parts of slurry up to about 1000 parts per million achieve the desired situation of freedom from bacterial contamination.
The Havens patent suggests that colloidal silica aquasols containing about 10-1000 parts per million of hexachlorophene can be protected from contamination by microorganisms. Addition of the hexachlorophene is intended to prevent discoloration, bad odor, and slime formation and increase the shelf life of colloidal silica sols to more than one year.
The Payne '431 and Nickerson patents are concerned with controlling bacterial growth in silica aquasols containing polyhydric alcohols. Payne 431' attempts to control and eliminate the growth of organisms such as aerobacter and pseudomonus bacteria, aspergillus niger mold, and troublesome desulfovibio and clostridia anaerobic bacteria by addition of a biocide. Typical biocides are glutaraldehyde, ethylenediamine, hydrogen peroxide and methyl p-hydroxybenzoate. Nickerson suggests that the addition of sodium pentachlorophenate will prevent or inhibit the darkening of silica aquasol containing polyhydric alcohol even in those instances where the silica aquasol contains sodium sulfate.
The Yoder and Roman et al. patents disclose the use of dialdehydes, such as glutaraldehyde, to control bacteria. While Michalski et al. '275 and McGahen disclose the use of formaldehyde to protect colloidal silica sols from bacteria growth. McGahen also discloses the use of 3,5-dimethyl tetrahydro 1,3,5,2-H-thiadiazine-2-thione as a microbiocide.
Although each of the aforementioned patents discloses various biocides for inhibiting bacterial growth in colloidal silicas, none of the aforementioned aquasols are satisfactory for use in the polishing of silicon wafers. That is, the aforementioned aquasols have unacceptable polishing rates for use in recirculated polishing systems.
The Payne '337, Payne '188, Payne '421, and Huff '612 disclose the use of various polishing rate accelerator amines added to conventional colloidal silica to form acceptable polishing agents. However, these patents are not concerned with either a low metals, low sodium colloidal silica or with a polishing agent which can be recirculated without increased microorganism or fungi growth.
Kohyama et al. discloses an aqueous dispersion of silicic anhydride having a silica particle size in the range between about 100 nm to 10,000 nm which is prepared from a dry method. PH controlling agents, such as amines, may be added to the silicic anhydride of Kohyama et al. The silicas prepared according to Kohyama et al. exhibit the following characteristics: (1) the particles are prepared by a dry method which can then be dispersed into a fluid medium, (2) the particles are not discrete but exist as condensed masses or aggregates, and (3) the aggregates settle with time and hence do not fit the historical definition of a silica colloid. Nor is Kohyama et al. concerned with low metals, low sodium colloidal silica or control of microorganism or fungi growth in recirculating systems.
The present inventors have found that although no microbiological growths are present in conventional colloidal silica at the outset, increased microbiological growth is observed during recirculation and dilution of the slurry. These microbiological growths are promoted when organic rate accelerators are used.
The present inventors undertook the task of examining the recirculation polishing system and developing a novel group of colloidal silica slurries which eliminate bacterial and fungi growth, maintain and, in some instances, increase the polishing rate of the system, and provide a polishing medium with extremely low values, particularly of Al, Fe, K, Na, and the other transition metals as described above.
Through lengthy experimentation, the present inventors have developed a novel group of low sodium, low metals colloidal silica slurries which are capable of inhibiting bacterial growth and enhancing the polishing rate of silicon wafers. These colloidal silica slurries are formed from a low metal, ammonium-stabilized silica sol that has particle sizes ranging from about 4 to about 130 nanometers. This sol has discrete spherical particles, and finds particular use in high quality investment casting, high technology refractories, catalyst applications, electronic polishing agents, and in high technology coating applications.
Additional advantages of the present invention shall become apparent as described below.